The present invention relates to a method and apparatus for registration control during the processing of workpieces. The invention is particularly useful in registration control during the production of images or other process performed with respect to printed circuit boards (PCBs) or other substrates, and is therefore described below with respect to such an application.
Registration control is important in many processing operations to be performed on workpieces. It is particularly important in processing operations requiring the precise matching of a film or an electronic image (i.e., the electronic representation of an image) to a given substrate. Examples of the latter processing operations include contact printing of patterns in multi-layer PCBs, manufacturing of integrated circuits in microelectronics, and plotting of pre-press tooling panels. For example, manufacturing multi-layer PCBs involves the fabrication, and then the stacking, of up to 20 or 30 layers, in which each layer (commonly referred to as an inner layer) has its own previously generated conductor or other (e.g., through holes) pattern.
Manufacturing such multi-layer PCBs encounters a number of registration problems, particularly the following: (1) registration of the image of each layer with respect to those in the other layers; (2) registration of an image on one side of a layer with respect to an image on the other side of the layer; (3) registration of the image plotted on each side of the layer with respect to other tasks involved in processing the layer or the board, such as outer-layer processes, drilling holes, etc.; and (4) registration of the image on each side of the layer with respect to existing pre-drilled holes through the layer. These registration problems become increasingly more difficult to overcome as the component-density of the boards, wafers, or other workpieces increases, the number of layers increases, and/or the size of the workpiece decreases.
Various techniques for overcoming these types of registration problems are described in U.S. Pat. Nos. 4,829,375, 5,136,948, 5,164,742, 5,274,394, 5,403,684, 5,453,777, 5,459,941, 5,500,801, 5,548,372, and 5,699,742.
Most of the known techniques generally involve reorienting the substrate with respect to the machine, and then plotting the image from the image file. However, such known techniques have a number of drawbacks, particularly when plotting on PCBs or other substrates under conditions producing variations in thickness in the layer, or variations in thickness or in length among a series of layers. For instance, when a substrate having significant thickness is mounted on a cylindrical drum which is rotated with respect to the printing or plotting elements, the outer surface of the substrate is under tension thereby increasing its length, whereas the inner surface of the substrate is under compression thereby decreasing its length. An image plotted from an image file on the tensioned outer surface of the loaded layer will shrink when the layer is unloaded; whereas an image plotted on the compressed inner surface will expand when the layer is unloaded. These effects produce scaling changes between the plotted image relative to the image file. These scaling changes, which depend on the thickness of the layer and the loading conditions, introduce registration problems when printing images from individual image files on a plurality of overlying layers or on the opposite faces of the layers.
Another registration problem is encountered when imaging two sides of a layer, one after the other in the plotting machines which can not plot the two sides of the layer simultaneously. This problem is present with respect to both rotary-drum and flat-bed imaging or plotting machines.
A still further registration problem is introduced by dimensional changes of the substrate during processing. For example, temperature variations during processing induce thermal expansion or contraction of the substrate. Thus, thermal changes during the processing of different layers produce dimensional changes which generate registration errors when the layers are stacked to produce the multiple-layer PCB. Dimensional changes are also introduced by mechanical deformation of the layers during processing (e.g., mechanical pre-cleaning), deformation due to stress release (e.g., due to heating in the resist-coating step or due to laminate deformation after copper etching), etc. These registration problems become much more difficult to overcome when the dimensional changes are local and therefore not correctable by a global compensation.